A transmitter apparatus for use in radio communications typically includes a power amplifying device. A high efficiency and a small distortion property are required for the power amplifying device. To achieve a high efficiency, Doherty amplifiers have been recently used for the power amplifying device. To reduce non-linear distortion, a distortion compensator circuit is also used together with the amplifier.
The Doherty amplifier unit includes a carrier amplifier and a peak amplifier. The carrier amplifier is designed to operate at a relatively high linearity with a class AB bias setting. The peak amplifier operates with a class C bias setting. The carrier amplifier constantly amplifies an input signal, while the peak amplifier does not amplify if an input signal has a low instantaneous power and amplifies an input signal at a peak power if the input signal has a high instantaneous power. The peak amplifier operates at a low power consumption when the instantaneous power of the input signal is low. When the instantaneous power is high, the output of the carrier amplifier and the output of the peak amplifier are synthesized into a high power output signal. In this way, the Doherty amplifier achieves a high power efficiency.
On the other hand, the linearity of the Doherty amplifier is insufficient. For example, distortion is generated in an amplified transmission signal due to AM-AM characteristics (input amplitude to output amplitude characteristics) and AM-PM characteristics (input amplitude to output phase characteristics). As illustrated in FIG. 10A, non-linear distortion is generated due to the non-linear characteristics of the AM-AM characteristics in a high input power region. As illustrated in FIG. 10B, sidelobes appear in the frequency characteristics, causing non-linear distortion. A signal is likely to leak into an adjacent channel in radio communications, thereby becoming noise on the adjacent channel. Communication quality is thus degraded. In particular, current transmitter devices used in radio communications have a high peak-to-average power ratio (PAPR), and linearity is required of the transmitter devices in a wide range of power. Due to insufficient linearity properties, the Doherty amplifier suffers from non-linearity problems.
In order to overcome the problem of the Doherty amplifier, a distortion compensator circuit such as a pre-distortion circuit is provided. In view of the non-linearity characteristics, a pre-distortion compensator circuit pre-distorts an input signal prior to amplification in order to overcome the non-linear distortion by multiplying the input signal by a compensation coefficient representing the inverse of non-linearity characteristics. When the input signal thus compensated is amplified by an amplifier, the characteristics inverse to the non-linearity characteristics and the non-linearity characteristics cancel each other out. The input signal prior to compensation is thus linearly amplified. As illustrated in FIG. 11A, the gain of the Doherty amplifier is lowered in a region where the amplitude of the baseband signal is large. By compensating for distortion with characteristics illustrated in FIG. 11B inverse to the gain characteristics, the gain is set to be constant regardless of the amplitude of the baseband signal.
FIG. 12 illustrates a transmitter apparatus 100 including a distortion compensator and a power amplifier based on the related art. The transmitter apparatus 100 includes a transmitter system 102 and a feedback system 130.
The transmitter system 102 includes baseband signal generator 104, serial-parallel converter 106, distortion compensator 108, digital-to-analog converter 110, orthogonal modulator 112, power amplifier 118, and antenna 120.
The baseband signal generator 104 generates, as a serial signal, a baseband signal composed of a digital in-phase (I) signal and a digital quadrature-phase (Q) signal. The serial-parallel converter 106 serial-to-parallel converts the two signals, i.e., the I signal and the Q signal. In order to compensate for the non-linearity of the power amplifier, the distortion compensator 108 pre-compensates for distortion by multiplying the I signal and the Q signal by a compensation coefficient. The digital-to-analog converter 110 digital-to-analog converts the compensated I signal and Q signal. The orthogonal modulator 112 orthogonally modulates the analog I signal and Q signal, thereby generating a transmission signal. The power amplifier 118 amplifies the transmission signal output from the orthogonal modulator 112 and outputs the amplified transmission signal. The antenna 120 emits a radiowave corresponding to the transmission signal amplified by the power amplifier 118.
The feedback system 130 includes directional coupler 132, frequency-converter 134, and analog-to-digital converter 138.
The directional coupler 132 separates part of the transmission signal output from the power amplifier 118 from the transmission signal, and the down-converter 134 frequency-converts the separated part of the transmission signal in a down-conversion operation. The analog-to-digital converter 138 analog-to-digital converts the input signal into a digital signal, thereby generating a feedback signal. The feedback signal output from the analog-to-digital converter 138 is supplied to the distortion compensator 108, which uses the feedback signal to generate a compensation coefficient.
Japanese Laid-open Patent Publication No. 2008-131186 discloses a power amplifier which works with low non-linearity distortion at a high efficiency and provides highly accurate distortion compensation corresponding to an output signal.
The disclosed power amplifier includes a Doherty amplifier, a pre-distortion unit, and a bias controller. The bias controller, having a bias control table storing information of an input power and information a gate voltage in a mapped state, controls a gate voltage of a peak amplifier by referencing the bias control table in response to a detected input power. The bias control table of the power amplifier maps the input voltage to a class C gate voltage when the input voltage is low, and maps the input voltage to a class AB gate voltage when the input voltage is high. The power amplifier also includes a distortion detector for detecting non-linear distortion in an output signal of the Doherty amplifier, and a controller. The controller adaptively updates the inverse of the characteristics of the pre-distortion unit and the bias control table in response to non-linear distortion detected by the distortion detector so that non-linear distortion is reduced.
The Doherty amplifier in the power amplifier unit disclosed above sometimes fails to output a sufficient output power in the vicinity of a peak output power. The transmitter apparatus illustrated in FIG. 12 may also suffer from the same problem.